1. Field of the Invention
The present invention relates to an image heating device that is suitable as a fixing device for fixing an unfixed toner image by heating a conductive belt directly or indirectly via a metal roller utilizing electromagnetic induction; an image forming apparatus, such as an electrophotographical apparatus or an electrostatic recording apparatus, using such an image heating device; an image copying machine using such an image forming apparatus; and a method for controlling temperature applicable to such an image heating device, an image forming apparatus, and an image copying machine.
2. Description of the Related Art
As image heating devices typically used for fixing devices, contact-heating type image heating devices such as roller-heating type devices and belt-heating type devices generally have been used.
In recent years, due to the demand for shorter warm-up time and reduced energy consumption, electromagnetic induction heating, by which rapid heating and high efficiency heating are likely to be attained, are attracting great attention. In the belt-heating type image heating devices, to shorten the warm-up time, a conductive belt having a smaller thermal capacity is used. A high-frequency current is applied to a magnetization coil to generate a high-frequency magnetic field, which causes an induced eddy current to be generated in the conductive belt, thereby causing Joule heat to be generated in the conductive belt itself. An unfixed toner image formed on a recording medium (paper, an OHP film, etc.) can be fixed after passing through a nip portion formed between a fixing roller and a pressure roller, which are pressed against with each other via the conductive belt that generates heat.
On the other hand, in the roller-heating type image heating devices, to shorten the warm-up time, a metal roller having a smaller thickness is used. A high-frequency current is applied to a magnetization coil to generate a high-frequency magnetic field, which causes an induced eddy current to be generated in the metal roller, thereby causing Joule heat to be generated in the metal roller. An unfixed toner image formed on a recording medium (paper, an OHP film, etc.) can be fixed after passing through a nip portion formed between the metal roller and the opposing pressure roller or between a fixing roller, to which heat conducted from the metal roller is transferred via a heat-resistant resin belt, and the opposing pressure roller.
In belt-type image heating devices (devices using a conductive belt or resin belt), a conductive belt having a small thermal capacity is heated through electromagnetic induction (direct heating of the belt), or a metal roller is heated through electromagnetic induction and the heat generated by the roller is conducted to a resin belt having a small thermal capacity (indirect heating of the belt). Thus, although the belt itself can be heated rapidly, a pressure roller having a large thermal capacity is heated slowly. Accordingly, in an early stage of the device operation, the temperature of the pressure roller is not sufficiently high while the belt already has reached a fixing temperature.
Even if the temperature of the pressure roller increases gradually with the temperature rise of the belt and reaches a desired fixing temperature afterward, a recording medium removes the heat from the pressure roller when passing through the nip portion. Consequently, glossiness decreases from the leading end toward the trailing end of the recording medium in the direction in which the recording medium is fed (hereinafter, referred to as “feed direction”), resulting in fixing failure in the worst case (irregularity in gloss in the feed direction).
By the way, if a new print request is issued after printing is started and before the formation of an electrostatic latent image by exposing means is completed, a print-processing sequence for forming another electrostatic latent image by the exposing means is executed continuously (continuous print mode). In this continuous print mode, as shown in FIG. 7A, a recording medium is fed when the necessary fixing temperature is maintained, similarly to the case where printing is performed on a plurality of recording media, and the heat is conducted to the recording medium from a paper feed portion at the center of the fixing belt and the pressure roller in the axial direction. In the fixing belt with a small thermal capacity, the temperature is liable to decrease from the end portions. However, an irregularity in temperature is not caused in the fixing belt because the difference in temperature between the end portions and the center whose temperature is decreased by the printing is small and the end portions are heated by a heat source by applying an electric power of, for example, about 500 W to compensate the temperature decrease.
However, in the case where a new print request is issued after the formation of an electrostatic latent image by the exposing means is completed and before paper ejection is completed, a print-start-processing sequence is executed immediately after a print-end-processing sequence is executed (semicontinuous print mode). In this semicontinuous print mode, as shown in FIG. 7B, the image heating device starts a temperature-maintaining operation, in which only a small electric power of, for example, about 300 W is used, immediately after the print-end-processing sequence is executed. As a result, an amount of the heat released from the fixing roller and the heat-generating roller becomes relatively large so that the temperature decreases at both ends of the fixing roller and the heat-generating roller in the axial direction. Thus, glossiness decreases at both ends of the recording medium in the axial direction of the heat-generating roller, resulting in fixing failure in the worst case (irregularity in gloss in the axial direction).